1. Field of the Invention
The present invention relates to an optical amplifying fiber applicable to an optical fiber amplifier and relates also to a forming method for a fiber grating in the optical amplifying fiber.
2. Description of the Related Art
An optical amplifier for directly amplifying an optical signal without having to convert the optical signal into an electrical signal is widely studied as one of key devices in a future optical communication system in many research institutes from a viewpoint that the optical amplifier is a bit-rate-free device in effect, so large-capacity transmission can be easily achieved, and from another viewpoint that simultaneous amplification of multiple channels can be performed. As one form of optical amplifiers, an optical amplifier using a single-mode optical fiber having a core doped with a rare earth element such as Er, Nd, or Yb (which will be hereinafter referred to as a doped fiber) is known. In operation, signal light to be amplified is input into the doped fiber to propagate therein, while pump light is introduced into the doped fiber in the same direction as the propagation direction of the signal light or in the opposite direction.
The optical amplifier using such a doped fiber is called an optical fiber amplifier, which has excellent features of no polarization dependence of gain, low noise, and low coupling loss to an optical transmission line. In putting this kind of optical fiber amplifier into practical use, it is required to ensure a wide wavelength band of signal light that can be amplified with a required gain and also ensure a high conversion efficiency of pump light to signal light.
FIG. 1 shows a schematic configuration of a conventional optical fiber amplifier. Reference numeral 2 denotes an Er doped fiber. Signal light input from an input port 4 and pump light output from a pumping source 6 are combined together by a multiplexer 8 to propagate in the Er doped fiber 2. In the Er doped fiber 2, the pump light is converted into signal light, and the signal light is gradually amplified along the Er doped fiber 2. The amplified signal light is output from an output port 10. Reference numerals 12 and 14 denote optical isolators.
In the above optical fiber amplifier, a conversion efficiency of pump light to signal light is reduced by amplified spontaneous emission (ASE) generated in the Er doped fiber 2. As shown in FIG. 2, ASE 18 is a component different from signal light 16, and the generation of ASE causes a reduction in amplification efficiency of signal light. Therefore, in an optical fiber amplifier, ASE must be efficiently removed to increase the conversion efficiency of pump light to signal light (the amplification efficiency of signal light).
Conventionally, a two-stage amplifier configuration as shown in FIG. 3 has been proposed to remove ASE. Signal light from an input port 4 and pump light from a pumping source 6 are combined together by a multiplexer 8 to forward propagate in Er doped fibers 2 and 20. On the other hand, pump light from a pumping source 22 is combined with the propagating signal light by a multiplexer 24 to backward propagate in the Er doped fiber 20. The pump light is converted into signal light in the Er doped fibers 2 and 20, and the amplified signal light is output from an output port 10. An optical band-pass filter 30 for transmitting the signal light and the pump light and removing ASE is provided between the Er doped fibers 2 and 20. Reference numerals 12, 14, 26, and 28 denote optical isolators. In such a two-stage optical fiber amplifier, ASE can be efficiently removed as shown in FIG. 4.
However, the two-stage optical fiber amplifier as shown in FIG. 3 requires many optical components, causing an increase in cost. Further, since the optical isolators 14 and 26 and the optical band-pass filter 30 are inserted between the two Er doped fibers 2 and 20, these optical components invite insertion loss even at a wavelength of signal light, thus causing a reduction in amplification efficiency of signal light.
In recent years, a fiber grating directly formed in an optical fiber as an optical filter has been developed to be put into practical use. However, a usual fiber grating forms an optical filter for controlling a transmission quantity or reflection quantity of light at an arbitrary wavelength, and it is therefore expected that if the fiber grating is used in an Er doped fiber having a gain of 30 dB or more, the gain is decreased because of a resonance phenomenon.